This method is supplied with an encrypted byte array, src, and an associated page number. Your method must decrypt first num_bytes
bytes from the src byte array and store the result into the tgt byte array. tgt is then used for data operations within your
application.

Any algorithm used must preserve the size of the data (encrypted data is the same length as the original data) and must be
able to decrypt partial pages (page 0 is first read and decrypted as a 128 bytes long page and then read and decrypted as
a full size page). The src array must not be modified.